The parapatric speciation proposes the emergence of new species from the reproductive isolation of two subpopulations that are found side by side. It is one of the three basic models of speciation, and conforms to an "intermediate" state between allopatric and sympatric models..
This theory implies the speciation in populations distributed in contiguous areas and that there is a moderate flow of genes between both regions. As there is a certain degree of isolation between the two subpopulations, each of these can increase the levels of genetic independence.
Over time, species can develop reproductive isolation mechanisms and the speciation process will be complete.
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Often any topic of discussion in evolutionary biology begins with the contributions of the famous British naturalist Charles Darwin..
In his masterpiece, The origin of species, Darwin proposes the mechanism of natural selection, and postulates - among other things - how new species can be formed by the gradual action of this mechanism, over long periods of time..
But what is a species? This question has been of great study and controversy for biologists. Although there are dozens of definitions, the most widely used and accepted concept is the biological concept of species, formulated by Ernst Mayr.
For Mayr, a species is defined as: "groups of interbreeding natural populations that are reproductively isolated from other groups." A critical point in this definition is the reproductive isolation between the groups that we call species.
In this way, a new species is formed when individuals belonging to two divergent populations do not recognize each other as potential mates..
Depending on the geographic context where speciation occurs, the authors use a classification system that includes three main models: allopatric, sympatric and parapatric speciation..
If the origin of the new species involves total geographic isolation (due to the emergence of a geographic barrier, such as a river or a mountain), the speciation is allopatric. If the species are formed in the same geographic area without any separation, it is sympatric speciation.
An intermediate model is parapatric speciation, where new species emerge in continuous geographic regions. We will describe this intermediate model in detail below..
It is important to mention that the distinction between the three types of speciation may not be clear and they overlap each other..
In parapatric speciation, the division of two biological "subpopulations" occurs that are located next to each other, without any geographical barrier that prevents the flow of genes between the two demos (a "demo" is another term widely used in the literature for refer to populations).
Parapatric speciation can occur in the following way: initially, a population is homogeneously distributed in a certain geographical area. Over time, the species evolves a pattern of "clina".
This clinal model was proposed by Fisher in 1930. Although it is the traditional model, there are other proposals - such as speciation "stepping-stone"
A cline is a phenotypic gradient that occurs in the same species - for example, in terms of body size: individuals range from large to small sizes.
The origin of the clina can occur due to an abrupt geographical change. Thanks to the change, some forms manage to adapt to the conditions on one side, while the remaining population adapts to the other side..
Between each of the limits, a hybrid zone would be formed, where the members of each side of the new geographic gradient come into contact and there is gene flow between both subpopulations. However, now the species on each "side" can be recognized as separate entities..
These two forms can receive different taxonomic names, and can be classified as races or as subspecies.
A zone of tension may form in the hybrid zone, which favors the speciation process. In this area, the formation of hybrids is disadvantageous - that is, hybrids have a biological suitabilitysmaller than parent species.
Suppose that an individual is homozygous dominant for a certain trait (AA), and is adapted to one side of the geographical area. On the other side are homozygous recessive individuals (aa), adapted to that region.
If a cross occurs in the hybrid zone between the two “races” or “subspecies” and the hybrid between both (in this case, the heterozygous Aa) has a lower biological suitability or fitness, it is a zone of tension. According to empirical evidence, almost all known hybrid zones fall within the definition of tension zone.
Thus, natural selection will favor selective mating between each of the variants that live in continuous geographic regions. That is, those on the left will reproduce each other and the same will happen on the right side.
Although the theoretical bases of parapatric speciation make it a possible and attractive model, the evidence is relatively little and not conclusive..
There is not enough evidence to illustrate every step of the process. However, the model is not completely ruled out and it may happen in certain cases.
Grass Anthoxanthum odoratum belonging to the Poaceae family, represents a very illustrative example of parapatric speciation.
Some of these plants live in areas where the soil is contaminated by a variety of heavy metals. In this way, only the variants of the grass that are able to tolerate contamination can grow in these regions..
In contrast, neighboring plants that do not live in contaminated soils have not undergone a selection process towards tolerance to heavy metals..
The tolerant and non-tolerant forms are close enough to fertilize each other (a requirement for the speciation process to be considered parapatric). However, both groups have developed different flowering times, establishing a temporary barrier to gene flow..
These two species of crows are distributed throughout Europe and are a classic example of a hybrid zone. C. corvix it is located further to the east, while its companion is located to the west, with a meeting point of both species in central Europe.
Although each species has its own phenotypic characteristics, in the area where they cross they can produce hybrids. Crossbreeding is a sign that the speciation process between the two ravens has not yet been completed and the reproductive isolation is not fully established..
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